The present invention relates to an improvement of a ball groove profile of a ball screw and, more particularly, to a ball groove profile of a screw shaft of a ball screw which can improve the durability thereof.
Shown in FIG. 5 is an exemplary ball groove profile of a screw shaft of a conventional ball screw in vertical cross section. This ball groove profile shown in FIG. 5 is an enlargement of a vertical sectional view of a ball groove 1a of a screw shaft 1 in a so-called tube type ball screw shown in FIG. 6 in which ball circulation tubes 3 are arranged in a nut 2 with which the screw shaft 1 is engaged through a plurality of balls. The ball groove la spirally formed on the screw shaft 1 is such that a land portion 4, which is a cylindrical outer diameter portion of the screw shaft, and an arcuate ball rolling portion 5 whose radius is r are connected through chamfered portions 6 (land shoulder portions), which are linearly inclined surfaces.
A preload is generally applied to the ball screw to improve rigidity against a load applied thereto in the axial direction and insure positional accuracy thereof. For example, a device for applying a preload such as shown in FIG. 7 (U.S. Pat. No. 4,177,690) is available. In this example, the lead of a ball groove 2a of the nut 2 is slightly shifted with respect to the ball groove 1a of the screw shaft 1. After a ball 7 rolls within the thus shifted grooves 1a, 2a, the ball 7 moves into the ball circulation tube 3 after being scooped by a scooping end portion 3a of the ball circulation tube 3, so that the ball 7 returns to grooves 1a, 2a. According to this conventional example, stoppage of the ball 7 at the scooping end portion 3a of the ball circulation tube is prevented by the chamfered portions 6 which help the ball 7 preloaded within both grooves 1a, 2a move upward while releasing the ball 7 slightly in the axial direction, and catching of the ball at the time the ball returns into both grooves 1a, 2a from the ball circulation tube 3 is prevented by the chamfered portions 6. In this conventional example, it is preferable to linearly chamfer the corners with an angle xcex1 shown in FIG. 5 ranging from 30 to 60xc2x0; it is ideal to linearly chamfer the corners with the angle xcex1 ranging from 45 to 50xc2x0. As a result of such profile, a point of intersection P1 between each chamfered portion 6 of the ball groove 1a of the screw shaft and the arcuate ball rolling portion 5 is formed like a projection.
An arcuately chamfered portion 8 shown in FIG. 8 is also known instead of the linearly chamfered portion 6. The arcuately chamfered portion 8 is mainly employed for a so-called piece circulation type ball screw having such circulation pieces 9 as shown in FIG. 9. This example is characterized as connecting the cylindrical outer diameter portion (land portion) 4 of the screw shaft 1 and the semi-circular ball rolling portion 5 whose radius is r through the arcuately chamfered portions 8 whose radius of curvature is R. To connect both portions 4, 5 and the chamfered portions 8 so as to be substantially continuous, the radius of curvature R of each arcuately chamfered portion is set to a value 40% the radius r of the ball 7 or less. Further, the point of intersection P1 between each arcuately chamfered portion 8 of the ball groove 1a of the screw shaft and the arcuate ball rolling portion 5 is still formed like a projection, although the angle of the projection is more obtuse than that shown in FIG. 5.
It should be noted that each of Y1/2 of FIG. 5 and Y2/2 of FIG. 8 denotes half the difference between the outer diameter of the screw shaft 1 and the diameter of the central circle of the ball 7 passing through the center O of the ball 7 within the ball groove 1a. In the cases of these conventional ball groove profiles, both Y1 and Y2 are set to comparatively large values, such as 10% of the diameter of the ball 7 or more.
It should also be noted that xcex8 denotes an angle indicating either the position of the point of intersection P1 between the ball rolling portion 5 and each linearly chamfered portion 6 or the position of the point of intersection P1 between the ball rolling portion 5 and each arcuately chamfered portion 8. The angle xcex8 is generally set to a value ranging from 65 to 70xc2x0.
When the ball screw is operated, the ball 7 rolls within a spiral load ball rolling passage formed of the ball groove 1a of the screw shaft and the ball groove 2a of the nut confronting the ball groove 1a, riding over the load ball rolling passage and is scooped by a ball circulation section such as the ball circulation tube 3 or the circulation piece 9, and sent into the load ball rolling passage again. This circulation process of the ball is repeated. The tracks of balls circulating in the ball screw are dispersed within a certain range. Machining/fabrication errors, operating speeds, and other operating conditions of the ball screw are responsible for the dispersion in track, which is therefore inevitable.
Because of the dispersion in the track of the ball circulation, it often happens that the ball 7 within the ball circulation section first collides with an area proximate the point P1 of FIG. 5 or FIG. 8 and then is sent into the load ball rolling passage.
However, in the conventional ball groove profiles of the screw shaft of the ball screw such as shown in FIGS. 5 and 8, the point P1 is projecting (the radius of curvature is small although the chamfered portion 8 is arcuate in the case of FIG. 8), which makes stress caused by the ball 7 colliding with an area near such a point large. Particularly, recent ball screws are subjected to large impact stresses due to high-speed operation, which damages the vicinity of the point P1, which is the land shoulder portion of the screw shaft of the ball screw. This shortens the life of a ball screw.
The present invention has been made in view of the problem noted above. Accordingly, the object of the present invention is to provide a ball screw capable of fulfilling the needs of an extended life under high-speed operation by improving the ball groove profile of the screw shaft of the ball screw.
To achieve the above object, the present invention is applied to a ball groove profile of a ball screw in which, in a vertical section, an outer diameter portion of the screw shaft and a substantially semi-circular ball rolling portion of the ball groove are connected to each other through arcuate portions and a radius of curvature of each arcuate portion ranges from a value between half a radius of a ball rolling in the ball groove or more to twice the radius of the ball or less, at least each arcuate portion being continuously and smoothly connected to the ball rolling portion.
Here, the outer diameter of the screw shaft may be almost equal to or slightly smaller than a diameter of a central circle of the ball fitted with the ball groove arranged on the outer diameter of the screw shaft; and a difference between the outer diameter of the screw shaft and the diameter of the central circle of the ball ranges from 10% of the diameter of the ball or less.
The land shoulder portions of the shaft of the ball screw are made arcuate and the radius of curvature of such arcuately formed land shoulder portion is set to a value as large as half to twice the radius of the ball to allow the land shoulder portions to be connected continuously and smoothly to the arcuate surface of the ball rolling portion. As a result of this profile, there is no projection as at the point P1 in the conventional examples. Accordingly, the stress concentration is reduced even if impact derived from the collision of the ball is applied to the land shoulder portions, so that the land shoulder portions are not damaged under high-speed operation.
In addition, the outer diameter of the screw shaft is set to a value almost equal to or slightly smaller than the diameter of the central circle of the ball fitted with the ball groove arranged on the outer diameter of the screw shaft, and the difference between the outer diameter of the screw shaft and the diameter of the central circle of the ball is set to 10% of the diameter of the ball or less. As a result of this profile, the collision of the ball with the point of intersection P2 between the arcuate portion and the land portion can be prevented even if the radius of curvature of the arcuate portion is increased.